Over recent years the use of liquid crystal display devices has spread rapidly not only in information and telecommunications equipment but in electrical equipment in general. Especially in mobile equipment, use is often made, in order to reduce power consumption, of the reflective type of liquid crystal display device, which does not require a backlight or sidelight as the transmissive type does. (Below, “backlight, etc.” is used to include both backlight and sidelight.) However, reflective liquid crystal display devices take the external light as their light source and consequently are poorly visible in dark places such as room interiors. Accordingly, recent times have seen the development of liquid crystal display devices that use a frontlight (disclosed in Japanese Laid-Open Patent Publication No. 2002-131742) or that combine qualities of both the transmissive type and the reflective type (disclosed in Japanese Laid-Open Patent Publication No. 2001-350158).
A reflective liquid crystal display device employing a frontlight is able, for example, to display images in dark places by lighting up the frontlight, while in light places it can utilize the external light to display images, without lighting up the frontlight. Hence there is no need to keep the frontlight permanently lit, which means that the power consumption can be drastically reduced. A semi-transmissive liquid crystal display device has within each of its pixels a transmissive portion equipped with a transparent electrode and a reflective portion equipped with a reflective electrode, and hence is able to display images in dark places by lighting up its backlight, etc., and making use of the transmissive portions of its pixel areas, while in light places it can utilize external light at its reflective portions to display images, without lighting up the backlight, etc. Thus with this type too there is the merit that the power consumption can be drastically lowered as the backlight, etc., does not need to be permanently lit.
In reflective and semi-transmissive liquid crystal display devices such as described above, the visibility of the liquid crystal display screen will vary with the intensity of the external light. This means that in order to obtain good visibility of the liquid crystal display screen, the end users must themselves perform the bothersome tasks of deciding whether or not the external light intensity is at a level where the backlight, etc., or frontlight should be lit, and of lighting, turning down or turning off the backlight, etc., or frontlight accordingly. There is also the prominent problem that in some cases the backlight, etc., or frontlight will be lit unnecessarily when the external light is of ample brightness, which will increase wasteful power consumption so that in mobile equipment such as mobile telephones the battery will become depleted early.
A well-known related art invention deals with such problems by providing the liquid crystal display device with photosensors; the brightness of the external light is sensed by the photosensors and the backlight, etc., is controlled to turn on and off according to the results of such sensing (disclosed in Japanese Laid-Open Patent Publication No. 2002-131719).
The liquid crystal display device set forth in disclosed in Japanese Laid-Open Patent Publication No. 2002-131719 below deploys, on a substrate of the liquid crystal display panel, a light sensing component that has a photosensor. A thin film transistor (TFT) is used as the photosensor. This TFT is created simultaneously with the liquid crystal display panel's TFTs. Via detection of the TFT photosensor's photosensor's light leakage current, the backlight is automatically turned on and off according to the ambient brightness.
Another liquid crystal display device, set forth in disclosed in Japanese Laid-Open Patent Publication No. 2003-215534 below, uses a photodiode as photosensor and, according to the ambient brightness, supplies current of assured temperature to a light-emitting diode serving as backlight.
A further device, set forth in disclosed in Japanese Laid-Open Patent Publication No. 2004-007237 below, makes a light-emitting diode serving as backlight or operation indicating means for the equipment serve also as a photosensor, and controls lighting of the backlight by using for the light-emitting diode the electromotive force appropriate to the ambient brightness.